An electro-optic device such as a liquid crystal display (Liquid Crystal Display: hereinafter referred to as “LCD”) includes a thin film transistor substrate (hereinafter referred to as “TFT substrate”) using a thin film transistor (Thin Film Transistor: hereinafter referred to as “TFT”) as a switching device.
A semiconductor device such as a TFT has a characteristic of low power consumption and a thin profile. Therefore, by utilizing such a characteristic of the semiconductor device, application to a flat panel display has been made, replacing a CRT (Cathode Ray Tube).
In an LCD which is one example of a flat panel display, generally, a liquid crystal layer is provided between a TFT substrate and a counter substrate. On the TFT substrate, TFTs are arranged, for example, in a matrix. Polarizing plates are provided outside such a TFT substrate and a counter substrate. Note that in transmissive and semi-transmissive LCDs, a back light unit is provided outside the TFT substrate or a counter substrate. Meanwhile, in a color display LCD, for example, a color filter of one color or two or more colors is provided on the counter substrate.
Typical structure of a TFT substrate for an LCD is disclosed, for example, in FIG. 1 of Patent Document 1. The TFT substrate has bottom-gate back-channel type TFTs, and pixel electrodes electrically connected to the TFTs are formed on a top layer. This structure can be manufactured using five photolithography processes.
Conventionally, in a switching device of a TFT substrate for a liquid crystal display device, amorphous silicon (Si) is generally used as a semiconductor active layer (channel layer). Furthermore, in recent years, development of a TFT using an oxide semiconductor for an active layer has been carried out actively. Since an oxide semiconductor has mobility higher than mobility of amorphous silicon, the oxide semiconductor has an advantage of implementing a small and high-performance TFT.
As an oxide semiconductor, a zinc oxide (ZnO)-based material and a material in which gallium oxide (Ga2O3), indium oxide (In2O3), oxidized tin (SnO2), or the like are added to zinc oxide are mainly used. This technology is disclosed, for example, in Patent Documents 2 and 3.
In a TFT using an oxide semiconductor for a channel layer, in a case where hydrogen in the channel layer is present between lattices, the hydrogen will produce disorder of structure and reduce TFT characteristics such as mobility. Also, it is said that hydrogen in the channel layer serves as an electron donor, and in that case, carrier density that contributes to characteristics will rise. Meanwhile, in a case where hydrogen terminates uncombined bonds, carrier density will decrease. In addition, in a case where hydrogen diffuses from other layers into the channel layer when a TFT operates, one of above phenomena will occur and characteristics will vary. Thus, hydrogen in the channel layer affects initial characteristics and reliability of a TFT.
Meanwhile, for example, in Patent Document 4, a technology of improving TFT characteristics by controlling discharge and diffusion of hydrogen and oxygen through a total of three times of heat treatments in a manufacturing process is disclosed. This is to increase an on-state current and mobility by eliminating excessive hydrogen in the channel layer through first heat treatment after forming the channel layer, next supplying oxygen to the channel layer from an insulating film containing oxygen and reducing oxygen deficiency through second heat treatment after forming the insulating film containing oxygen above the channel layer, then supplying hydrogen to the channel layer from an insulating film containing hydrogen through third heat treatment after forming the insulating film containing hydrogen on the insulating film containing oxygen and terminating defects or uncombined bonds.